Radiance-general Digest, Vol 52, Issue 19

Dear all,
  I am trying to obtain irradiance value on the surfaces of building blocks under daylight but encounter the following problems. Could you shed some light on how they can be solved?
  1. Uneven distribution of the uniform sky
  I produced a uniform sky using the –u option under the gensky function. As measured from the fisheye view of the sky (both angular and hemispheric), however, the radiance of the sky slightly decreases towards the edge of the “circle” (please see attached). Is there a way to improve this situation? Here are the parameters that I used for rendering the sky:
  > Rpict –vf view.vf –ab 0 –aa 0 –ar 128 –ad 4096 – as 1024 sky.oct
   
  2. Different value obtain in each run
  I tried to render some building blocks (with homogeneous color and reflectance) and then measure the irradiance of a point on the ground with rtrace. Different values return even if the inputs are identical. The discrepancy can be as much as 5%. Like the command line printed above, ambient parameters had already been set at their maximum values (except for -ab which is set at 1). Is there a way to obtain irradiance values with a smaller margin of error?
   
  Thank you very much for your help!
   
  Kam

Dear Kam,

As John Mardaljevic points out, the change at the edges of the sky is due to the "mixing function" designed to reduce variance at the horizon between the sky and the ground. You can also avoid its appearance without modifying skyfunc.cal by setting -g 1.0 in gensky, I believe.

As for the repeatability of the results, John's reference is a good one:

  http://www.radiance-online.org/pipermail/radiance-general/2006-December/004147.html

I'm grateful to him for digging this up -- I would have tried to answer it all over again...

Best,
-Greg

P.S. It is better not to send attachments to the list.

···

From: Kam Shing Leung <[email protected]>
Date: June 16, 2008 11:23:43 PM PDT

Dear all,
I am trying to obtain irradiance value on the surfaces of building blocks under daylight but encounter the following problems. Could you shed some light on how they can be solved?
1. Uneven distribution of the uniform sky
I produced a uniform sky using the –u option under the gensky function. As measured from the fisheye view of the sky (both angular and hemispheric), however, the radiance of the sky slightly decreases towards the edge of the “circle” (please see attached). Is there a way to improve this situation? Here are the parameters that I used for rendering the sky:
> Rpict –vf view.vf –ab 0 –aa 0 –ar 128 –ad 4096 – as 1024 sky.oct

2. Different value obtain in each run
I tried to render some building blocks (with homogeneous color and reflectance) and then measure the irradiance of a point on the ground with rtrace. Different values return even if the inputs are identical. The discrepancy can be as much as 5%. Like the command line printed above, ambient parameters had already been set at their maximum values (except for -ab which is set at 1). Is there a way to obtain irradiance values with a smaller margin of error?

Thank you very much for your help!

Kam